阿拉善地块南缘镁铁-超镁铁岩形成时代及地球化学特征

Petrogenetic epoch and geochemical characteristics of mafic-ultramafic rocks on the southern margin of Alxa massif in northern China

阿拉善地块南缘是镁铁超镁铁岩体的重要分布区 ,自北而南可分为北大山岩带、龙首山岩带和北海子岩带。北大山岩带以野芨里含长辉石橄榄岩侵入体为代表 ,Sm_Nd岩石矿物等时线年龄为 773.1± 10 .8Ma,εNd(t)=+0 .8～ +10 .8,ΣREE为 7.5 9× 10 -6～ 2 0 .34× 10 -6;龙首山岩带以金川二辉橄榄岩体和藏布台单辉橄榄岩为典型代表 ,Sm_Nd岩石矿物等时线年龄分别为 15 0 8± 31Ma和 15 11± 16 8Ma ,基本为同期幔源产物 ,金川岩体平均ΣREE为 3.33× 10 -6～ 115 .5 2× 10 -6,εNd(t) =- 1.9～ - 4 .3,藏布台εNd(t) =+2 .6～ +2 .9,差异显著 ;北海子岩带岩体分布有限 ,其中辉长岩体Sm_Nd岩石矿物等时线年龄为 14 4 0± 2 2 0Ma ,εNd(t) >0 ,ΣREE为 179.6 4× 10 -6～ 2 0 7.76× 10 -6。龙首山岩带和北海子岩带可能同为中元古代早期超地幔柱作用下地幔派生岩浆的产物 ,εNd(t)值的差异反映了各自岩体成岩过程的不同 ;野芨里岩体形成于新元古代 ,是亏损地幔的产物 ,与金川等岩体的形成环境和物质来源有显著不同。

The southwestern margin of Alxa massif in northern China, one of the major provinces of mafic-ultramafi rocks, can be divided into Beidashan, Longshoushan and Beihaizi rock belts. Generally speaking, the Beidashan rock belt is represented by Yejili feldspar-bearing pyroxene peridotite, whose Sm-Nd rock-mineral isochron age is 773.1±10.8 Ma,εNd(t) values are +0.8～+10.8, and ΣREE are 7.59×10^(-6)～20.34×10^(-6). The Longshoushan rock belt is represented by Jinchuan lherzolite and Zangbutai pyroxene peridotite, whose Sm-Nd rock-mineral isochron ages are 1?508±31 Ma and 1?511±168 Ma respectively, implying mantle products formed almost at the same time. Jinchuan intrusion has ΣREE of 3.33×10^(-6)～115.52×10×10^(-6) and εNd(t) of -1.9～-4.3, but Zangbutai has εNd(t) of +2.6～+2.9, showing obvious differences. The Beihaizi rock belt is spread limitedly, whose gabbro Sm-Nd rock-mineral isochron age is 1?440±220 Ma,εNd(t)>0,andΣREEare179.64×10^(-6)～207.76×10^(-6).TheLongshoushanandBeihaizirockbeltswereboth probably formed by the early Middle Proterozoic magma derived from the lower mantle due to the action of superplume. Their εNd(t) difference indicates that every rock body has its unique petrogenetic process. Yeliji rock body is the product of Later Proterozoic depleted mantle, whose formation environment and origin are remarkably different from those of Jinchuan rock body.